In recent years, lead-based piezoelectric materials such as lead zirconate titanate (Pb(Zr,Ti)O3) and lead-free piezoelectric materials containing no lead are used for mechano-electrical transducers to be applied to drive elements, sensors and the like. Such piezoelectric materials, being formed into thin films on substrates of silicon (Si) or the like, are expected to be applied to MEMS (Micro Electro Mechanical Systems) elements.
In manufacture of MEMS elements, high-precision processing using semiconductor process technologies such as photolithography can be applied, and smaller and higher-density elements can thus be obtained. In particular, fabrication of elements all together at a high density on a Si wafer having a relatively large diameter of 6 or 8 inches can significantly reduce the cost as compared to single-wafer manufacturing in which elements are individually produced.
Furthermore, as a result of reducing the thickness of piezoelectric materials and making devices in the form of MEMS devices, the efficiency of mechano-electrical transduction is improved, which also produces a new added value such as improvement in device sensitivity and characteristics. For example, heat sensors are reduced in thermal conductance as a result of being made in the form of MEMS devices and can thus be improved in measurement sensitivity, and inkjet heads for printers can achieve high-resolution patterning as a result of increased density of nozzles. In addition, a high piezoelectric constant d31 is required for a piezoelectric thin film necessary for such devices.
When a piezoelectric thin film is used as a MEMS drive element, the piezoelectric thin film needs to be formed to have a thickness of 3 to 5 μm, for example, which may depend on the device to be designed, so as to satisfy a required displacement generating force. For forming a piezoelectric thin film on a substrate made of Si or the like, chemical film forming methods such as CVD (Chemical Vapor Deposition), physical methods such as sputtering and ion plating, and liquid phase growth method such as the sol-gel method are known, and it is important to find conditions for film formation to obtain films having required properties depending on these film forming methods.
The material used for a piezoelectric thin film is often crystals of PZT, that is, lead (Pb), zirconium (Zr), titanium (Ti), and oxygen (O). PZT exhibits a good piezoelectric effect with an ABO3 perovskite structure as illustrated in FIG. 8, and thus needs to be a single-phase perovskite. Conversely, a piezoelectric thin film having a low crystallinity and being increased in crystals with a pyrochlore structure and in amorphous regions has low piezoelectric properties. Since Pb easily evaporates in PZT film formation, conditions for film formation need to be set carefully to obtain perovskite crystals.
The shape of unit cells of PZT crystals having the ABO3 perovskite structure varies depending on the ratio of Ti and Zr that are atoms in a B site. Specifically, the PZT crystal lattice is tetragonal when the amount of Ti is large, and the PZT crystal lattice is rhombohedral when the amount of Zr is large. When the molar ratio of Zr to Ti is approximately 52:48, both of these crystal structures are present, and a phase boundary having such a composition ratio is referred to as a MPB (Morphotropic Phase Boundary). Since maximums of piezoelectric properties such as the piezoelectric constant, the polarization value, and the dielectric constant are obtained with this MPB composition, piezoelectric thin films having the MPB composition are actively used.
Furthermore, in addition to using the perovskite crystallinity and the MPB composition, proper control of the crystal orientation of piezoelectric thin films is also important for increasing the piezoelectric constant. For example, orientations of Pb-based perovskite crystals are (100), (110), (111), etc., among which the (001) orientation using polarization aligned in the direction of electric field application and the (100) orientation using effects of domain rotation are effective in increasing the piezoelectric constant d31 as illustrated in FIG. 9. In FIG. 9, thick black arrows indicate the direction of polarization. In this case, if a normal piezoelectric strain ΔX1 when an electric field is applied in the (001) direction to a domain having tetragonal polarization in the (001) direction and a piezoelectric strain ΔX2 when the same electric field is applied in the direction of (001) to a domain having tetragonal polarization in the (100) direction are compared, the piezoelectric strain ΔX2 resulting from rotation of the domain 90° from the (100) direction to the (001) direction is larger than the normal piezoelectric strain ΔX1.
Since the crystal orientation of a piezoelectric thin film is strongly influenced by an underlying layer, control of the crystal orientation of the underlying layer is very important in addition to conditions for forming the piezoelectric thin film. Thus, a method of performing high-temperature Pt film formation under an oxygen atmosphere to forma Pt lower electrode in the (100) orientation and forming a Pb-based perovskite piezoelectric thin film thereon in the (100) orientation, and a method of forming a film of a Pt lower electrode in the (001) orientation on a single crystal substrate of (001) MgO or the like and forming a Pb-based perovskite piezoelectric thin film thereon in the (001) orientation are proposed. It is difficult, however, to put these methods to practical use in terms of reliability and productivity.
Thus, in Patent Literature 1, for example, a buffer layer for controlling the crystal orientation of PZT is formed between an electrode made of a set of columnar grains made of Pt and having a cross-sectional diameter of not smaller than 20 nm and not larger than 30 nm and PZT in the form of a Pb-based perovskite ferroelectric thin film. The buffer layer is made of perovskite lead lanthanum titanate (PLT) having a (001) crystal orientation ratio of 50%. In this manner, a perovskite PLT film in the (001) orientation is formed on an electrode to form PZT in the (001) orientation on the PLT.